1. Field of the Invention
The present invention relates to a polymer scale preventive agent useful in polymerization of a monomer having an ethylenically unsaturated double bond, a polymerization vessel effective in preventing polymer scale deposition, and a process of producing a polymer using said vessel.
2. Description of the Prior Art
In a process of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymer deposits on the inner wall surface and so forth in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer and operators may be exposed thereto, which may cause physical disorders in the operators.
Heretofore, as methods for preventing polymer scale deposition on the inner wall surface and so forth of a polymerization vessel in polymerization of a monomer having an ethylenically unsaturated double bond, methods in which suitable substances are coated on the inner wall surface, etc. as a scale preventive agent, have been known.
As suitable substances for use as a polymer scale preventive agent include, there have been disclosed, for example, particular polar organic compounds (Japanese Patent Publication (KOKOKU) No. 45-30343(1970)), a dye or pigment (Japanese Patent Publication (KOKOKU) Nos. 45-30835(1970) and 52-24953(1977)), an aromatic amine compound (Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976)) and a reaction product of a phenolic compound and an aromatic aldehyde (Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980)).
In the meantime, where polymeric products obtained by polymerization are processed into formed products such as sheets, the resulting formed products are required to have high whiteness. That is, when a polymeric product is formed into a sheet or the like without any addition of a coloring agent, the resulting formed product is more or less colored. Such coloration is called initial coloration, which is desired to be as low as possible. In the case of vinyl chloride polymers, for example, the formed products are desired to have a luminosity index L in the Hunter's color difference equation described in JIS Z 8730(1980) of 70 or more.
However, when polymeric products obtained by polymerization in a polymerization vessel provided on its inner wall surface with a coating comprising a prior-art polymer scale preventive agent, as exemplified typically by the dyes and pigments described in Japanese Patent Publication (KOKOKU) Nos. 45-30835(1970) and 52-24953(1977), the aromatic amine compounds described in Japanese Pre-examination Patent Publication (KOKAI) No. 51-50887(1976) and the reaction products of a phenolic compound and an aromatic aldehyde described in Japanese Pre-examination Patent Publication (KOKAI) No. 55-54317(1980), are processed into formed products such as sheets, the resulting formed products are low in whiteness, namely, high in initial coloration. In the case of vinyl chloride polymers, specifically, the luminosity index (L value) of the formed products is normally 65 or below. This is presumably because the polymer scale preventive agents according to the prior art are, in many case, colored substances. In other words, the coating comprising a colored substance as a polymer scale preventive agent will be incorporated into the polymerization mass through dissolution or peeling, leading to the low whiteness.
Furthermore, the polymer scale preventive agents according to the prior art have the drawback that, although they can prevent the deposition of polymer scale on the areas in the liquid phase inside the polymerization vessel, they are unable to prevent the scale deposition in the vicinity of the interface between the gas phase and the liquid phase inside the polymerization vessel.
Once polymer scale is deposited in the vicinity of the interface between the gas phase and the liquid phase, the deposited scale will grow gradually as polymerization runs are repeated, and at last it is peeled from the inner wall, etc. and incorporated into the polymeric product. If .epsilon. the polymeric product containing the polymer scale is processed into formed products such as sheets or the like, the polymer scale causes increase in fish eyes in the formed products, lowering seriously the quality thereof.
Moreover, the prior art polymer scale preventive agents generally include poisonous substances as exemplified typically by deleterious substances such as aniline, nitrobenzene, formaldehyde, etc. among the polar compounds described in the above-mentioned Japanese Patent Publication (KOKOKU) No. 45-30343(12970), and pigments containing a heavy metal such as chromium or lead among the pigments described in Japanese Patent publication (KOKOKU) No. 45-30835(1970). Besides, the dyes described in Japanese Patent Publication (KOKOKU) Nos. 45-30835(1970) and 52-24953(1977) include some dyes that involve concern about carcinogenesis. Therefore, use of these substances may cause problems in safety of operators.